From previous patents and applications originating with our group, there are known and have been described films obtained from alpha-olefin polymers of a prevailingly isotactic structure; more particularly obtained from polypropylene consisting essentially of isotactic macromolecules and prepared with stereospecific catalysts.
The mechanical, protective and optical characteristics of such films, and in particular of polypropylene films, are such as to make them particularly suited to be used in the packaging field.
The impermeability of said films to gases and vapors may not be as high as required for some applications such as, for example, the packaging of certain foodstuffs which require a particularly high impermeability.
With the aim of improving the impermeability to gases and vapors and the flavors of the olefinic films, it is a usual technique to coat said films with suitable materials.
Generally, the coating is achieved by extrusion of a lacquer onto the carrier film in the molten state (extrusion coating); by "lamination" of two films to each other, with or without interposed adhesive substances; or by spreading the coating substance dissolved in a suitable solvent, possibly together with a primer that will facilitate the adhesion of the coating to the carrier. Often, the primer or "anchoring agent", is substituted or is accompanied by a pre-treatment of the supporting film with chemical agents, with non-piercing electrical discharges, with flame or the like.
As materials suitable for the coating of the polyolefinic films there have been described and suggested numerous compounds, prevailingly polymeric compounds, which in general, are applied in the form of solutions in organic solvents. The use of organic solvents, which subsequently are removed from the coated film by evaporation, make it necessary to install apparatuses for the recovery of the solvents, and devices keeping the work places free of vapors. Moreover, it is necessary to use new measures for preventing the pollution of the surrounding atmosphere as well as the pollution of the wash water.
There have been suggested polymeric coating materials that may be applied to the films in the form of aqueous dispersions.
The possibility of applying the coating to polyolefinic films from aqueous solutions or dispersions allows to attain a substantial saving on the process, both because no complex and expensive equipment is required for the recovery of the solvents and for the solution of problems connected with the environmental hygiene and pollution, and because it makes the process less complex.
The films coated with the known water-soluble or water-dispersible polymeric materials have, however, little or unsatisfactory impermeability to gases, vapors and flavors, so that they are not suited for the packaging of foodstuffs.